The present invention relates to a method for simultaneous identification and correction of errors due to magnetic perturbations and to misalignments in the measurements of a magnetometer mounted on board a vehicle, as well as to a device for implementing this method and to various systems using said device.
It is known that a magnetometer mounted on board a vehicle, in particular an aircraft, is generally subjected to magnetic perturbations and that it is not rigorously aligned relative to the reference coordinate system of the vehicle, which makes the magnetic field measurements taken by this magnetometer inaccurate.
Essentially four types of perturbation can be categorized:
three types of magnetic perturbation, namely, more specifically: PA0 installation errors. It is known that the ideal positioning of the magnetometer corresponds to perfect alignment of the reference coordinate system of the magnetometer relative to that of the vehicle. However, because of manufacturing tolerances, this perfect alignment is never achieved, which causes perturbation of the vector measurements taken. PA0 Hm represents the perturbed field measured by the magnetometer; PA0 H represents the effective value of the magnetic field; PA0 [B] is a symmetric matrix taking into account the perturbations generated by the "soft irons", the absence of an antisymmetric part being due to the precise and suitable installation of the magnetometer on the vehicle; and PA0 H.sub.B represents a perturbing field due to the perturbations generated by the "hard irons". PA0 taking a large number of measurements of the magnetic field during variations in the heading and the attitude of the vehicle; PA0 determining the ellipsoid best corresponding to the measurements taken; and PA0 calculating, from the ellipsoid thus determined, the components of the matrix [B] and of the perturbating field H.sub.B. The model then obtained makes it possible to calculate the effective field H from subsequent measurements taken by the magnetometer and subjected to magnetic perturbations. PA0 in a first step, a method similar to that of the previous method is used to determine a measurement conserving constancy of the norm of the field, said measurement being then affected only by a misalignment; and PA0 in a second step, the asymmetric component of the transformation matrix similar to the previous matrix [B] is determined from the variations in the vertical component of the magnetic field, this step being implemented by using attitude measurements. PA0 a theoretical model correcting the errors of the magnetometer is defined in the form: EQU Hc=[A].Hm+a.Hm'+Hp, PA0 the corrected field Hc is assumed to be the terrestrial field defined in the vehicle reference frame, so that: EQU [A].Hm+a.Hm'+Hp=[M].H PA0 at least one measurement Hm of the magnetic field is taken using the magnetometer; PA0 the derivative Hm' of said measured magnetic field Hm is calculated; PA0 the coefficients of the change of frame matrix [M] are determined; PA0 an error vector E with components Ex, Ey and Ez is defined, defined by the equation EQU E=[M].H-([A].Hm+a.Hm'+Hp); PA0 a composite error E.sup.2 =Ex.sup.2 +Ey.sup.2 +Ez.sup.2 is defined; PA0 a system of equations formed by n equations of the type (.differential..SIGMA.E.sup.2)/.differential.ci=0, i=1 to n, (.differential..SIGMA.E.sup.2)/.differential.ci corresponding to the partial derivative of the sum of the composite errors, for all the measurements, .SIGMA.E.sup.2 with respect to a coefficient ci, the various coefficients ci representing the coefficients to be determined in said theoretical model, namely the coefficients of [A], of a and of Hp, as well as at least one component of the effective magnetic field H; PA0 said system of equations is solved so as to obtain said coefficients ci, including said component of the effective magnetic field H; and PA0 the corrective model obtained from said coefficients is used to correct the errors due to magnetic perturbations and misalignments in the measurements of said magnetometer. PA0 higher precision in the measurements, confirmed by aircraft flight trials; and PA0 completely free installation of the magnetometers on the vehicle, making it possible to simplify the fastening device, which saves weight and reduces the manufacturing cost. PA0 a first calculation module, receiving the magnetic field Hm measured by the magnetometer and capable of determining the time derivative Hm' of said measured magnetic field Hm which varies as a function of the angular position of the vehicle; PA0 a second calculation module capable of calculating the coefficients of the change of frame matrix [M], which are used for implementing the invention, from the attitudes and, if necessary, the heading of the vehicle; and PA0 a main calculation module, connected to said first and second calculation modules and capable of determining said corrective model and said effective magnetic field.
so-called "soft iron" perturbations due to the presence in proximity to the magnetometer of ferromagnetic materials which, although magnetized, deflect the field lines; PA1 so-called "hard iron" perturbations, due to the presence in proximity to the magnetometer of magnetized and/or electrically conductive materials through which direct currents flow; and PA1 magnetic fields which are generated by the eddy currents created by magnetic flu variations in the metal structures of the vehicle; and PA1 Hc is the corrected field, PA1 [A] is a matrix to be determined, PA1 a is a coefficient or a matrix to be determined, PA1 Hp is a perturbing field to be determined, PA1 Hm is the value measured by the magnetometer in the vehicle reference frame of the magnetic field, and PA1 Hm' is the time derivative of said measured magnetic field Hm which varies as a function of the angular position of said vehicle; PA1 H the effective value of the magnetic field in a base reference frame, and PA1 [M] a change of frame matrix from said base reference frame to said vehicle reference frame;
In order to obtain exact measurements, it is therefore necessary, if appropriate, to identify the errors generated by these perturbations and to correct them. Various methods are known for making such a correction.
According to a first known method, which requires the magnetometer to be installed with precision on the vehicle in order to used, a magnetic perturbation model of the form Hm=[B].H+H.sub.B is defined, in which:
It will be noted that, in the absence of magnetic perturbations, the matrix [B] then representing the identity matrix and the perturbating field H.sub.B being zero, the modules of the measured magnetic field is constant and the end describes a sphere. Under the effect of perturbations, this sphere is transformed into an off-center ellipsoid.
Said first known correction method specifies that the following operations are carried out:
However, as indicated above, this method requires a particular and very precise arrangement of the magnetometer on the vehicle, which requires specific and expensive manufacture and/or adjustment. In addition, the precision obtained by this method is unsatisfactory. These drawbacks are partially solved by a second known method, which is subject to much less stringent constraints than the previous method with regard to the arrangement of the magnetometer on the vehicle, this second known method being implemented in two successive steps:
This second known method has numerous drawbacks. In particular, implementing the two aforementioned steps presupposes a complex and lengthy maneuvering procedure of the vehicle, during which the measurements used are taken.